The invention relates to a monolithically integratable circuit configuration for driving a semiconductor switch in a switched-mode power supply, having the following features:
the circuit configuration has a first and a second supply potential terminal;
a control unit for producing control pulses for the semiconductor switch according to a variable oscillator signal, whereby the duration of the individual control pulses depends on a first control signal and on a second control signal;
a measurement system for producing the second control signal, which is dependent on the load current of the semiconductor switch.
Circuit configurations of this type are used to regulate the output voltage, or the output power, of a switched-mode power supply.
The power consumed or output by a switched-mode power supply is determined by, among other factors, the duration of the periodically produced control pulses that cause a closing of the semiconductor switch for the duration of the control pulses, thus causing a flow of the load current. The regulation of the duration of the control pulses, which are usually generated in sync with the oscillator signal, takes place in the circuit configuration dependent on a first and on a second control signal. The first control signal depends on, among other factors, the output voltage, or the output power, of the switched-mode power supply.
In the prior art circuit configurations, there thus takes place a closing of the semiconductor switch in time with the oscillator signal, whereby the semiconductor switch is opened again dependent on the course of the first and second control signal. The control pulses are usually selected such that they terminate when the first control signal is exceeded by the second control signal, through which the semiconductor switch is opened.
Switched-mode power supplies are used, among other things, for supplying power to monitors or television sets. So that electrical and magnetic scatter fields of the switched-mode power supply will not be able to cause disturbances of the picture, the switched-mode power supply is usually synchronized with the line frequency of the monitor. There exist a multiplicity of different standards and different screen resolutions. For this reason, a monitor must be able to be adapted to different line frequencies over a broad frequency range. As a lower limit, almost all monitors operate with a line frequency of 31.5 kHz, in order to ensure compatibility with the VGA standard in DOS mode.
In order to achieve a higher display screen resolution, as well as a higher vertical scanning frequency of the electron beam, higher line deflection frequencies must be used. With higher line deflection frequencies, the flickering of the monitor can be reduced. Currently, for 17xe2x80x3 monitors, the upper limit of the line frequency is 85 kHz. For 21xe2x80x3 monitors, this limit is 108 kHz. In the future, a further increase of the line deflection frequency is planned.
In order to be able to meet the requirements of the synchronization of the line frequency of the monitor with the oscillator frequency of the switched-mode power supply, the switched-mode power supply must operate over a broad frequency range, from 31.5 kHz up to approximately 120 kHz.
The power requirement of a monitor varies according to the size of the picture tube, between approximately 70 watts and 140 watts. For reasons of cost, as a rule a flyback converter is used. Due to the lower radiation of electromagnetic interference fields, the flyback converter is preferably operated in delta current operation. Here, the primary winding of the transformer is periodically connected with the rectified input voltage between the first and second supply potential terminal until the flow of current through the primary winding, starting from the value zero, has reached a value that depends on a control signal. Subsequently, the flow of current in the primary winding is interrupted, and the overall magnetically stored energy flows off to the load at the secondary side via rectifying diodes. The power emission of a synchronized flyback converter in delta current operation is dependent on the maximum current in the primary winding and on the frequency of the power supply, i.e., the line deflection frequency.
This has the consequence that at the highest line frequency, the switched-mode power supply in a monitor can emit a significantly higher power than at the lowest line deflection frequency. However, the actual power requirement of a monitor in fact hardly depends on the selected line deflection frequency.
If, in operation with a high line deflection frequency, a fault occurs in which there arises a considerable additional power loss, without a lowering of the output voltage of the switched-mode power supply and the possibility of detecting the error in this way, the monitor can cause a fire.
It is therefore known from the prior art to limit the emitted power of the switched-mode power supply by monitoring the secondary current in the secondary transformer winding and providing a feedback loop in order to control the semiconductor switch. In addition, it is known to control the primary current dependent on the line frequency. However, none of these solutions can be realized in monolithically integrated form. In addition, the power limitation operates in a relatively imprecise fashion.
It is accordingly an object of the invention to provide a clocked current supply, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a circuit configuration that can be monolithically integrated that prevents an excessive emission of power of the switched-mode power supply in all operating states, in order to prevent damage to the power supply, as well as to the additional components supplied by it. In addition, the power regulation is to take place with a high degree of precision.
With the foregoing and other objects in view there is provided, in accordance with the invention, a monolithically integratable control circuit for driving a semiconductor switch in a switched-mode power supply with a first and a second supply potential terminal, which comprises:
a control unit for generating control pulses for the semiconductor switch based on an oscillator signal having a variable oscillator frequency, wherein a duration of individual drive pulses depends on a first control signal that is dependent on an output voltage of the switched-mode power supply, and on a second control signal;
a measurement system connected to said control unit for generating the second control signal in dependence on a load current of the semiconductor switch;
a power regulation system connected to said control unit, said power regulation system receiving the oscillator signal, generating a third control signal inversely proportional to a square root of the oscillator frequency, and supplying the third control signal to said control unit.
The invention is based on the knowledge that the power emission in delta current operation in a flyback converter increases as the square of the maximum current in the primary winding, and proportional to the frequency, i.e., to the line deflection frequency. The power regulation system provided in the switched-mode power supply has the effect that the maximum peak current in the primary winding is taken back in a manner inversely proportional to the square root of the frequency. The power regulation system therefore provides the control unit with a third control signal, which, dependent on the already-selected frequency, controls the semiconductor switch in such a way that the maximum peak current (load current) in the primary winding is influenced in a manner corresponding to the above-named relationship.
For this purpose, and in accordance with an added feature of the invention, the power regulation system has the following features:
a pulse generator that receives the oscillator signal;
a series circuit of a source of current and a charge storage unit, said circuit being situated between a third and a fourth supply potential terminal;
a series circuit of a MOS diode and a controllable switch, said circuit being connected in parallel to the charge storage unit;
the switch is controlled according to the pulse generator, whereby the pulse-duty ratio is dependent on the oscillator signal.
The third control signal is thereby picked off at the node between the current source, the charge storage unit, and the MOS diode.
The charge storage unit is charged with a constant current via the current source, and is periodically discharged for a fixed time interval via the MOS transistor wired as a diode. The period duration with which the switch is controlled thereby corresponds to the period duration of the switched-mode power supply. The difference of the capacitor voltage resulting on average and the inception voltage of the MOS diode yields a value that can be used to limit the primary peak current. This value represents the third control signal, which is supplied to the control unit. The advantage of this system is that the MOS diode has a current-voltage characteristic curve having a quadratic shape. The voltage that is connected via the MOS diode is consequently a square root function of the current flowing through it. Thus, the maximum peak current in the primary winding can be taken back in a manner inversely proportional to the square root of the frequency.
A further advantage of the invention is that the current source and the charge storage unit are already present in flyback converter switched-mode power supplies known from the prior art, since in this way what is known as a xe2x80x9csoft startxe2x80x9d is realized. The primary peak current is limited dependent on the voltage that is present at the charge storage unit. A flyback converter switched-mode power supply having a soft start is known for example from the textbook xe2x80x9cSchaltnetzteile,xe2x80x9d by Hirschmann and Hauenstein, published by Siemens A G, 1990, pages 179-90.
In accordance with an advantageous construction, the control unit has a comparator to which the first control signal is supplied at a first inverting input, the second control signal is supplied at a second inverting input, and the third control signal, which is dependent on the load current of the semiconductor switch, is supplied at a non-inverting input.
Through this, it is possible to carry out a frequency-dependent power limitation. However, in the case of normal operation, the semiconductor switch is controlled via the first control signal, which represents a voltage signal picked off from the secondary side.
In accordance with a further advantageous construction, the power regulation system has a temperature compensation device that subtracts a predetermined value from the voltage at the node point and supplies it to the comparator as a third control signal. The voltage at which a current begins to flow according to the quadratic current-voltage characteristic of the MOS diode varies dependent on the ambient temperature. Due to the fact that the power regulation system has a temperature compensation device, this offset can be almost avoided.
For this purpose, the temperature compensation device has a semiconductor switch and a current source that is tied to the fourth supply voltage potential. The node point between the semiconductor switch and the current source is connected to the first inverting input of the comparator.
The semiconductor switch of the temperature compensation device has a performance that depends strongly on temperature, so that a value dependent on the temperature is subtracted from the voltage at the node point, and the offset is avoided.
In a further advantageous construction, means are provided in the pulse generator that compensate the temperature-dependent characteristic of the MOS diode of the power regulation system.
Just as the inception voltage of the MOS diode varies with temperature, the steepness of the current-voltage gradient also varies dependent thereon. The temperature compensation system of the pulse generator modifies the pulse-duty ratio of the switch of the power regulation system in a manner depending on the temperature. Through this, the precision of the power regulation of the overall switched-mode power supply can be considerably improved. The temperature compensation is noticeable above all when the ambient temperature has a value greater than 40xc2x0 C.
In accordance with a concomitant feature of the invention, the pulse generator has a charge store to which a semiconductor switch controlled by the oscillator signal is connected in parallel, and to which a temperature-dependent semiconductor switch is connected downstream. The produced pulse duration is dependent on the duration, in order to charge the charge storage unit from the third supply potential to a first predetermined reference potential.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a clocked power supply, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.